M16CXO - M 16 (Eagle Nebula) Chandra X-Ray Point Source Catalog

Mechanisms regulating the origin of X-rays in young stellar objects and the
correlation with their evolutionary stage are under debate. Studies of the
X-ray properties in young clusters allow us to understand these mechanisms.
One ideal target for this analysis is the Eagle Nebula (M 16), with its
central cluster NGC 6611. At 1750 pc from the Sun, it harbors 93 OB stars,
together with a population of low-mass stars from embedded protostars to
disk-less Class III objects, with age <= 3 Myr. The authors study an archival
78 ks Chandra/ACIS-I observation of NGC 6611 and two new 80-ks observations
of the outer region of M 16, one centered on the Column V and the other on a
region of the molecular cloud with ongoing star formation. They detect 1755
point sources with 1183 candidate cluster members (219 disk-bearing and 964
disk-less), studying the global X-ray properties of M 16 and comparing them
with those of the Orion Nebula Cluster. The authors also compare the level of
X-ray emission of Class II and Class III stars and analyze the X-ray spectral
properties of OB stars. Their study supports the lower level of X-ray
activity for the disk-bearing stars with respect to the disk-less members.
The X-ray luminosity function (XLF) of M 16 is similar to that of Orion,
supporting the universality of the XLF in young clusters. Eighty-five percent
of the O-type stars of NGC 6611 have been detected in X-rays. With only one
possible exception, they show soft spectra with no hard components,
indicating that mechanisms for the production of hard X-ray emission in O
stars are not operating in NGC 6611.

The determination of the absorption corrected X-ray luminosity (LX), as
well as the plasma temperature (kT) and hydrogen column density (NH),
requires the analysis of the X-ray spectra. The authors fit the observed
spectra with thermal plasma (with both one and two temperatures) and
power-law models. They use the APEC ionization-equilibrium thermal plasma
code, assuming the sub-solar elemental abundances of Maggio et al. (2007,
APJ, 660, 1462). The absorption was treated using the WABS model. The
one-temperature (1T) thermal model was applied to all the sources with more
than 25 counts, while the two-temperature (2T) thermal model was applied to
each source with more than 80 counts. The power-law model has been applied to
those sources with hard spectra for which the best-fit thermal model predicts
a plasma temperature kT > 5 keV. When more than one model has been used for a
given source, the authors chose the best model by the chi-squared probability
and visual inspection of the spectrum.

This table contains a description of the X-ray properties of 1754 sources
(one less than stated in the abstract of the reference paper) derived from
three Chandra/ACIS-I observations, together with a source classification
based on the optical and infrared properties of each source. Data come from
three ACIS-I observations (central or 'c', east or 'e', and north-east or
'ne') and many values are not averaged but presented for each observation as
indicated by the parameter prefixes 'c_', 'e_', and 'ne_', respectively.
Source detection has been performed with PWDetect, adopting a threshold
corresponding to 10 spurious detections.

The HEASARC has eliminated the 3 parameters describing the plasma temperature
of the second spectral component and its associated negative and positive
errors for sources in the north-east observation, as these were blank for all
entries in the original table as obtained from the CDS.

Source_Number
A running X-ray source number in order of increasing J2000.0
Right Ascension.

Name
The name for the X-ray source created by the HEASARC using the '[GCM2012]'
prefix (for Guarcello, Caramazza, Micela 2012) and the source number, in the
style recommended by the CDS Dictionary of Nomenclature of Celestial Objects.

Alt_Name
An alternative name for the X-ray source also created by the HEASARC using
the 'CXOU' prefix (for Chandra X-ray Observatory, Unregistered) and the
truncated J2000.0 equatorial coordinates, in the style recommended by the CDS
Dictionary of Nomenclature of Celestial Objects, e.g., 'CXOU JHHMMSS.S-DDMMSS'.

RA
The Right Ascension of the Chandra X-ray source in the selected equinox. This
was given in J2000.0 decimal degrees to a precision of 10-6 degrees in the
original table. ACIS Extract (AE) uses the source positions provided by the
source detection algorithm (PWDetect in this case). Once both the PSF and the
background for each source are evaluated, AE computes new source positions by
correlating the source images with the local PSF model. The user has the
possibility to use these new positions or retain the set of positions
provided by the source detection algorithm. As suggested by the AE manual,
the authors updated the positions of the sources observed with an off-axis
larger than 5 arcminutes (further coordinate corrections of Delta_RA = -0.18"
and Delta_Dec = -0.05" have been applied to cross-correlate the X-ray catalog
with the optical-IR catalog; see Section 2.2 of the reference paper).

Dec
The Declination of the Chandra X-ray source in the selected equinox. This was
given in J2000.0 decimal degrees to a precision of 10-6 degrees in the
original table. ACIS Extract (AE) uses the source positions provided by the
source detection algorithm (PWDetect in this case). Once both the PSF and the
background for each source are evaluated, AE computes new source positions by
correlating the source images with the local PSF model. The user has the
possibility to use these new positions or retain the set of positions
provided by the source detection algorithm. As suggested by the AE manual,
the authors updated the positions of the sources observed with an off-axis
larger than 5 arcminutes (further coordinate corrections of Delta_RA = -0.18"
and Delta_Dec = -0.05" have been applied to cross-correlate the X-ray catalog
with the optical-IR catalog; see Section 2.2 of the reference paper).

C_Off_Axis
The off-axis angle of the X-ray source from the pointing
direction for the east observation, in arcminutes.

E_Off_Axis
The off-axis angle of the X-ray source from the pointing
direction for the east observation, in arcminutes.

Ne_Off_Axis
The off-axis angle of the X-ray source from the pointing
direction for the north-east observation, in arcminutes.

C_Prob_No_Source
The probability associated with the null hypothesis that
there is no real source associated with the specified position in the central
observation. Only 21 sources have a probability value larger than the
threshold of 0.1 typically used with AE.

E_Prob_No_Source
The probability associated with the null hypothesis that
there is no real source associated with the specified position in the central
observation.

Ne_Prob_No_Source
The probability associated with the null hypothesis that
there is no real source associated with the specified position in the central
observation.

C_Significance
The photometric significance of the X-ray source in the
central observation computed as the ratio of
the net counts in 0.5-8 keV to the upper error in the net counts.

E_Significance
The photometric significance of the X-ray source in the
east observation computed as the ratio of
the net counts in 0.5-8 keV to the upper error in the net counts.

Ne_Significance
The photometric significance of the X-ray source in the
north-east observation computed as the ratio of
the net counts in 0.5-8 keV to the upper error in the net counts.

C_Raw_Counts
The total number of counts associated with the X-ray source in
the whole energy band (0.5-8.0 keV) in the central observation.

E_Raw_Counts
The total number of counts associated with the X-ray source in
the whole energy band (0.5-8.0 keV) in the east observation.

Ne_Raw_Counts
The total number of counts associated with the X-ray source
in the whole energy band (0.5-8.0 keV) in the north-east observation.

C_Counts
The net background-corrected counts associated with the X-ray
source in the whole energy band (0.5-8.0 keV) in the central observation.
Background events have been extracted for each source in circular annuli
centered on the sources, applying a mask for source counts defined in two
iterations (first a circular area covering 99% of the local PSF and
then with a more accurate mask region). The extraction regions of crowded
sources are reduced to avoid overlapping each other,
down to 40% of the local PSF in the most crowded regions.

E_Counts
The net background-corrected counts associated with the X-ray
source in the whole energy band (0.5-8.0 keV) in the east observation.
Background events have been extracted for each source in circular annuli
centered on the sources, applying a mask for source counts defined in two
iterations (first a circular area covering 99% of the local PSF and
then with a more accurate mask region). The extraction regions of crowded
sources are reduced to avoid overlapping each other,
down to 40% of the local PSF in the most crowded regions.

Ne_Counts
The net background-corrected counts associated with the X-ray
source in the whole energy band (0.5-8.0 keV) in the north-east observation.
Background events have been extracted for each source in circular annuli
centered on the sources, applying a mask for source counts defined in two
iterations (first a circular area covering 99% of the local PSF and
then with a more accurate mask region). The extraction regions of crowded
sources are reduced to avoid overlapping each other,
down to 40% of the local PSF in the most crowded regions.

C_PSF_Fraction
The fraction of the PSF of the X-ray source at 1.497 keV
corresponding to the extraction region in the central observation.
The extraction regions of crowded
sources are reduced to avoid overlapping each other,
down to 40% of the local PSF in the most crowded regions.

E_PSF_Fraction
The fraction of the PSF of the X-ray source at 1.497 keV
corresponding to the extraction region in the east observation.
The extraction regions of crowded
sources are reduced to avoid overlapping each other,
down to 40% of the local PSF in the most crowded regions.

Ne_PSF_Fraction
The fraction of the PSF of the X-ray source at 1.497 keV
corresponding to the extraction region in the north-east observation.
The extraction regions of crowded
sources are reduced to avoid overlapping each other,
down to 40% of the local PSF in the most crowded regions.

C_Count_Rate
The count rate of the X-ray source in the 0.5-8.0 keV band in
the central observation, in ct/s.

E_Count_Rate
The count rate of the X-ray source in the 0.5-8.0 keV band in
the east observation, in ct/s.

Ne_Count_Rate
The count rate of the X-ray source in the 0.5-8.0 keV band in
the north-east observation, in ct/s.

C_Median_Energy
The median photon energy of the X-ray source in the
0.5-8.0 keV band in the central observation, in keV.

E_Median_Energy
The median photon energy of the X-ray source in the
0.5-8.0 keV band in the east observation, in keV.

Ne_Median_Energy
The median photon energy of the X-ray source in the
0.5-8.0 keV band in the north-east observation, in keV.

C_NH
The value of the hydrogen column density NH, in atoms cm-2, in the
direction of the X-ray source in the central observation. This value is
estimated either from the spectral fitting or from quantile analysis,
as described in Section 3 of the reference paper.

C_NH_Pos_Err
The upper error in the hydrogen column density, in atoms
cm-2, in the direction of the X-ray source in the central observation.

C_NH_Neg_Err
The lower error in the hydrogen column density, in atoms
cm-2, in the direction of the X-ray source in the central observation.

E_NH
The value of the hydrogen column density NH, in atoms cm-2, in the
direction of the X-ray source in the east observation. This value is
estimated either from the spectral fitting or from quantile analysis,
as described in Section 3 of the reference paper.

E_NH_Pos_Err
The upper error in the hydrogen column density, in atoms
cm-2, in the direction of the X-ray source in the east observation.

E_NH_Neg_Err
The lower error in the hydrogen column density, in atoms
cm-2, in the direction of the X-ray source in the east observation.

Ne_NH
The value of the hydrogen column density NH, in atoms cm-2, in
the direction of the X-ray source in the north-east observation. This value is
estimated either from the spectral fitting or from quantile analysis,
as described in Section 3 of the reference paper.

Ne_NH_Pos_Err
The upper error in the hydrogen column density, in atoms
cm-2, in the direction of the X-ray source in the north-east observation.

Ne_NH_Neg_Err
The lower error in the hydrogen column density, in atoms
cm-2, in the direction of the X-ray source in the north-east observation.

C_kT_1
The plasma temperature of the first APEC thermal component, in keV,
for the X-ray source in the central observation. This value is estimated
either from the spectral fitting or from quantile analysis as described in
Section 3 of the reference paper.

C_kT_1_Pos_Err
The upper error in the plasma temperature of the first APEC
thermal component, in keV, for the X-ray source in the central observation.

C_kT_1_Neg_Err
The lower error in the plasma temperature of the first APEC
thermal component, in keV, for the X-ray source in the central observation.

E_kT_1
The plasma temperature of the first APEC thermal component, in keV,
for the X-ray source in the east observation. This value is estimated
either from the spectral fitting or from quantile analysis as described in
Section 3 of the reference paper.

E_kT_1_Pos_Err
The upper error in the plasma temperature of the first APEC
thermal component, in keV, for the X-ray source in the east observation.

E_kT_1_Neg_Err
The lower error in the plasma temperature of the first APEC
thermal component, in keV, for the X-ray source in the east observation.

Ne_kT_1
The plasma temperature of the first APEC thermal component, in keV,
for the X-ray source in the north-east observation. This value is estimated
either from the spectral fitting or from quantile analysis as described in
Section 3 of the reference paper.

Ne_kT_1_Pos_Err
The upper error in the plasma temperature of the first APEC
thermal component, in keV, for the X-ray source in the north-east observation.

Ne_kT_1_Neg_Err
The lower error in the plasma temperature of the first APEC
thermal component, in keV, for the X-ray source in the north-east observation.

C_Log_Lx
The logarithm of the adopted X-ray luminosity log(LX) of the
Chandra source in the 0.5-8 keV band, in erg/s, in the central observation.
See Section 4 of the reference paper for details on how LX is determined.

C_Log_Lx_Pos_Err
The upper error in the logarithm of the adopted X-ray
luminosity log(LX) of the Chandra source in the 0.5-8 keV band, in erg/s,
in the central observation.

C_Log_Lx_Neg_Err
The lower error in the logarithm of the adopted X-ray
luminosity log(LX) of the Chandra source in the 0.5-8 keV band, in erg/s,
in the central observation.

C_Log_Lx_Flag
This flag parameter is set to 'i' (for infinity, apparently)
when the negative error in the corresponding parameter is left blank.
The HEASARC believes that this should be interpreted as meaning that there
is no finite lower bound to the corresponding parameter value.

E_Log_Lx
The logarithm of the adopted X-ray luminosity log(LX) of the
Chandra source in the 0.5-8 keV band, in erg/s, in the east observation.
See Section 4 of the reference paper for details on how LX is determined.

E_Log_Lx_Pos_Err
The upper error in the logarithm of the adopted X-ray
luminosity log(LX) of the Chandra source in the 0.5-8 keV band, in erg/s,
in the east observation.

E_Log_Lx_Neg_Err
The lower error in the logarithm of the adopted X-ray
luminosity log(LX) of the Chandra source in the 0.5-8 keV band, in erg/s,
in the east observation.

E_Log_Lx_Flag
This flag parameter is set to 'i' (for infinity, apparently)
when the negative error in the corresponding parameter is left blank.
The HEASARC believes that this should be interpreted as meaning that there
is no finite lower bound to the corresponding parameter value.

Ne_Log_Lx
The logarithm of the adopted X-ray luminosity log(LX) of the
Chandra source in the 0.5-8 keV band, in erg/s, in the north-east observation.
See Section 4 of the reference paper for details on how LX is determined.

Ne_Log_Lx_Pos_Err
The upper error in the logarithm of the adopted X-ray
luminosity log(LX) of the Chandra source in the 0.5-8 keV band, in erg/s,
in the north-east observation.

Ne_Log_Lx_Neg_Err
The lower error in the logarithm of the adopted X-ray
luminosity log(LX) of the Chandra source in the 0.5-8 keV band, in erg/s,
in the north-east observation.

Ne_Log_Lx_Flag
This flag parameter is set to 'i' (for infinity, apparently)
when the negative error in the corresponding parameter is left blank.
The HEASARC believes that this should be interpreted as meaning that there
is no finite lower bound to the corresponding parameter value.

C_Log_Lx_Calc_Flag
This flag indicates how the X-ray luminosity of the
Chandra source in the central observation was calculated:

C_Spectral_Model
The spectral model used to fit the spectrum of the X-ray
source in the central observation, either thermal_1T, thermal_2T or power_law.
(See Overview above for more details on the spectral fitting.)

E_Spectral_Model
The spectral model used to fit the spectrum of the X-ray
source in the east observation, either thermal_1T, thermal_2T or power_law.
(See Overview above for more details on the spectral fitting.)

Ne_Spectral_Model
The spectral model used to fit the spectrum of the X-ray
source in the north-east observation, either thermal_1T, thermal_2T or
power_law. (See Overview above for more details on the spectral fitting.)

C_Ks_Prob_Constant
The K-S statistic for the null hypothesis that the
source is not variable in the 0.5-8keV band in the central observation.

E_Ks_Prob_Constant
The K-S statistic for the null hypothesis that the
source is not variable in the 0.5-8keV band in the east observation.

Ne_Ks_Prob_Constant
The K-S statistic for the null hypothesis that the
source is not variable in the 0.5-8keV band in the north-east observation.

C_kT_2
The plasma temperature of the second APEC thermal component, in keV,
for the X-ray source in the central observation. This value is estimated
either from the spectral fitting for those sources which are well-fit by a
thermal two-temperature plasma model.

C_kT_2_Pos_Err
The upper error in the plasma temperature of the second APEC
thermal component, in keV, for the X-ray source in the central observation.

C_kT_2_Neg_Err
The lower error in the plasma temperature of the second APEC
thermal component, in keV, for the X-ray source in the central observation.

E_kT_2
The plasma temperature of the second APEC thermal component, in keV,
for the X-ray source in the east observation. This value is estimated
either from the spectral fitting for those sources which are well-fit by a
thermal two-temperature plasma model.

E_kT_2_Pos_Err
The upper error in the plasma temperature of the second APEC
thermal component, in keV, for the X-ray source in the east observation.

E_kT_2_Neg_Err
The lower error in the plasma temperature of the second APEC
thermal component, in keV, for the X-ray source in the east observation.

C_Photon_Index
The photon index Gamma of the power-law model for the
source, if it is best fitted by such a model, in the central observation.

C_Photon_Index_Pos_Err
The upper error in the photon index Gamma of the
power-law model for the source in the central observation.

C_Photon_Index_Neg_Err
The lower error in the photon index Gamma of the
power-law model for the source in the central observation.

E_Photon_Index
The photon index Gamma of the power-law model for the
source, if it is best fitted by such a model, in the east observation.

E_Photon_Index_Pos_Err
The upper error in the photon index Gamma of the
power-law model for the source in the east observation.

E_Photon_Index_Neg_Err
The lower error in the photon index Gamma of the
power-law model for the source in the east observation.

Ne_Photon_Index
The photon index Gamma of the power-law model for the
source, if it is best fitted by such a model, in the north-east observation.

Ne_Photon_Index_Pos_Err
The upper error in the photon index Gamma of the
power-law model for the source in the north-east observation.

Ne_Photon_Index_Neg_Err
The lower error in the photon index Gamma of the
power-law model for the source in the north-east observation.